The present invention relates to improved treatment fluids for use in subterranean applications, and, at least in some embodiments, to micro-crosslinked gels and their associated methods of use. As used herein, the term “micro-crosslinked gel” refers to a gel that comprises small, substantially noncontiguous, substantially permanently crosslinked volumes, hereinafter referred to as “micro-domains.” These will be described more thoroughly herein. The term “gel,” as used herein and its derivatives refer to a viscoelastic or semi-solid, jelly-like state assumed by some colloidal dispersions.
Hydraulic fracturing is a process commonly used to increase the flow of desirable fluids, such as oil and gas, from a portion of a subterranean formation. Hydraulic fracturing operations generally involve placing a viscous fracturing fluid into a subterranean formation or zone at a rate and pressure sufficient to cause the formation or zone to break down with the attendant production of one or more fractures. The pressure required to induce fractures in rock at a given depth is known as the “fracture gradient.” Enhancing a fracture includes enlarging a pre-existing fracture in the formation. As the fracture is created or enhanced, a portion of the fluid contained in the viscous fracturing fluid leaks off into the formation, and a filter cake comprised of deposited gelling agent is built up on the walls of the fracture. Particulates, such as grains of sand, may be suspended in the fracturing fluid and introduced into the created fractures. As the viscous fracturing fluid leaks off into the formation, particulates aggregate in proppant packs within the fracture. The proppant packs function to prevent the fracture from fully closing upon the release of pressure, forming conductive channels through which fluids may flow to (or from) the well bore.
Gravel packing is another subterranean application that involves the use of particulates suspended in a viscous fluid. A “gravel pack” is used to at least partially reduce the migration of unconsolidated formation fines into the well bore. To form a gravel pack, particulate material, such as sand, is delivered downhole suspended in a viscous fluid. The fluid may then leak-off into the formation or be recovered from the well bore. Gravel packing operations commonly involve placing a gravel pack screen in the well bore neighboring a specified portion of the subterranean formation and packing the annulus between the screen and the subterranean formation with particulate materials. The particulates are sized to inhibit the passage of formation fines through the gravel pack with produced fluids. In some instances, a screenless gravel packing operation may be performed.
In some situations, hydraulic-fracturing operations and gravel-packing operations may be combined into a single operation to stimulate production and to reduce the production of unconsolidated formation particulates. Such treatments are often referred to as “frac-pack” operations. In some cases, these treatments are completed with a gravel-pack screen assembly in place with the fracturing fluid being pumped through the annular space between the casing and screen. In such a situation, the fracturing operation may end in a screen-out condition creating an annular gravel pack between the screen and casing.
In subterranean applications, a treatment fluid should have a sufficiently high viscosity to suspend the particulates as the treatment fluid is injected into the well bore and formation fractures. As used herein, the term “treatment fluid” refers generally to any fluid that may be used in a subterranean application in conjunction with a desired function and/or for a desired purpose. The term “treatment fluid” does not imply any particular action by the fluid or any component thereof. A “particulate-laden treatment fluid” is a treatment fluid that comprises particulates such as proppant or gravel.
Gelling agents have heretofore been utilized to gel a base fluid, producing a treatment fluid with adequately high viscosity. These gelling agents can be biopolymers or synthetic polymers that, when hydrated and at a sufficient concentration, are capable of forming a more viscous fluid. Common gelling agents include polysaccharides (such as xanthan, guar gum, diutan, succinoglycan, scleroglucan, etc.), synthetic polymers (such as polyacrylamide, polyacrylate, polyacrylamide copolymers, and polyacrylate copolymers), and surfactant gel systems. Guar and derivatized guar polymers, such as hydroxypropylguar, are economical water soluble polymers which can be used to create high viscosity aqueous fluids. Surfactant gel systems also have been used in subterranean formations at these temperatures, but such systems can be expensive, can be sensitive to impurities, and may require hydrocarbon breakers. To increase the viscosity of the resultant treatment fluid, the gelling agents may be crosslinked through an applicable crosslinking reaction comprising a crosslinking agent. Conventional crosslinking agents usually comprise a metal complex or other compound that interacts with at least two polymer molecules to form a “crosslink” between them. Oftentimes, for example to decrease pipe friction and/or provide a “lipping gel” (defined below) for particulate suspension and transportation, it is generally preferred that crosslinking occur very close in time to when the gel is introduced to a desired portion of a formation.
Typically, after a high viscosity, particulate-laden treatment fluid is pumped into a well bore and the particulates are placed as desired, the treatment fluid will be caused to revert into a low viscosity fluid. This process is often referred to as “breaking” the fluid. The treatment fluid “breaks,” or decreases in viscosity, so that it can more easily be removed from the well, while leaving a proppant and/or gravel pack in the fracture. Breaking the gel is most commonly accomplished by adding a breaker to the treatment fluid prior to pumping it into the well bore. Breakers, such as oxidizers, enzymes, and acid release agents, have been used successfully. Depending on the crosslinking agent used, a fluid may be broken by “delinking” the crosslinks between the gelling agent molecules. In such instances, this may be useful because oftentimes the fluid can be recovered, recrosslinked, and reused, whereas more typical “broken” fluids cannot.
Historically, to be considered to be a suitable particulate-laden treatment fluid for use in many subterranean applications, it has been believed that the crosslinked gels need to exhibit sufficient viscoelastic properties, in particular relatively high viscosities (e.g., at least about 300-500 cP at a shear rate of 100 sec−1). One aspect of such gel behavior may be described in the art as “lipping,” which may be distinguishable from freely pouring when poured out of a container. One example of a “lipping” gel is illustrated in FIG. 1A, contrasted with one example of a “nonlipping” gel illustrated in FIG. 1B. “Lipping” as used herein refers to a gel having sufficient homogeneous, three-dimensional elasticity on a macroscopic level with very few, if any, distinct micro-domains. Such lipping gels may be referred to herein as “macro-crosslinked gels.”
One problem associated with some gelled treatment fluids is that they are not able to continue to suspend particulates at elevated temperatures for a desirable length of time. This can pose significant challenges to subterranean applications that operate at elevated in situ temperatures. To offset this tendency, some have tried increasing the concentration of gelling or crosslinking agents in the treatment fluids. However, this has not proven to be an optimal solution because of the increased friction pressures that results. This makes pumping the treatment fluid more difficult and may increase the cost of the job. Additionally, the filter cake produced from the treatment fluids is often thick, tacky, and relatively difficult to remove from the walls of the well bore and the surfaces of the fractures in the subterranean formation.
Another disadvantage associated with using some gelled treatment fluids is that they can leave residue in the formation that can impact the productivity of the well. Residue may result from the continuing presence of the insoluble portions of the gelling agents, such as proteins, cellulose and fibers, in the pores of the subterranean zones being treated as well as gravel packs and proppant packs in the zones.
In subterranean applications such as well bore cleanout, the objectives are primarily focused on displacement of drilling fluids or other fluids occupying the well bore and removal of drilling fluid residue and other contaminants occupying the well bore. In this regard, a displacement fluid, sometimes called a spacer fluid, is used. Oftentimes, coiled tubing may be used to place the displacement fluid in the well bore. The displacement fluid may be a gelled treatment fluid. It is generally believed that the displacement fluid should be similar in density to the drilling or other fluid occupying the well bore to prevent substantial commingling of these fluids during the displacement process. Additionally, the displacement fluid often contains an agent to aid in removing contaminants adhering to the well bore walls as well as certain solids which may be loosely in residence in the well bore. Often, this also results in the removal of well bore-fill material, such as sand, scale, or organic materials, and other debris from the well bore.
In many subterranean applications, it is desirable for a treatment fluid to inhibit the amount of leakage of the liquid phase of a treatment fluid into the formation matrix. Fluid loss control agents are often used to control the process and avoid potential reservoir damage. This is also thought to be helpful in maintaining fracture width and length.
In some subterranean applications, it may be desirable to divert the flow of treatment fluids. In other subterranean applications, it may be desirable to divert the flow of formation fluids, such as preventing the excessive production of formation brine. Since fluids may tend to follow the path of least resistance, fluid flow may be diverted for example, by invading the higher permeability portions of the formation with a fluid that has high viscosity at low shear rates.